Currently, IEEE 802.11ah defines a local area network protocol especially for sub 1 GHz carrier frequencies. The main requirements of IEEE 802.11ah include a large coverage area (e.g., up to 1 km), a physical (PHY) layer data rate of at least 100 kbps, a maximum aggregate multi-station data rate of 20 Mbps, the use of orthogonal frequency division multiplexing (OFDM) PHY modulation, and support of a number of associations beyond 2007 for outdoor applications.
Overhead is an issue under the 802.11ah protocol. The physical layer of 802.11ah uses a slower clock than that used in the normal 802.11 protocol. Therefore, each symbol length under 802.11ah is ten times as long as under the normal 802.11 protocol.
For a downlink traffic transmission scheme in the 802.11 protocol, at every beacon frame, an access point (AP) broadcasts a traffic indication map (TIM) that has information on whether the downlink packet is buffered to each of the stations. If the downlink packet is buffered, a station, after reading the TIM, transmits a power save (PS)-Poll message indicating that the station is awakened and ready to receive downlink data packet. If the access point is ready to transmit buffered data, the access point immediately transmits downlink data packets. If the access point is not ready to transmit buffered data, the access point transmits an acknowledgment packet, and the access point will transmit the buffered data shortly.